| 英文摘要: | Following the April 25, 2015 Gorkha magnitude 7.8 and the May 12, 2015 magnitude 7.3 earthquakes, the largest earthquakes to occur in the central segment of the Himalaya in 80 years, a collaborative team from Oregon State University, U.C. Riverside, Stanford University, and the University of Texas at El Paso deployed an NSF RAPID-funded aftershock array of 45 seismic stations (41 broadband and short-period seismometers, 14 strong motion sensors) across the Main Himalayan Thrust (MHT) fault where the large earthquakes occurred. The array, known as the Nepal Array Measuring Aftershock Seismicity Trailing Earthquake (NAMASTE) remained in place for 11 months, allowing for a unique data set right over the active Himalayan thrust system. In this proposal, this same collaborative team will analyze the newly collected data set specifically to study the geometry of faults, including the MHT fault, the composition of the crust and upper mantle in the region, the rupture process of this earthquake sequence, and the broader geotectonic setting in the Himalaya and southern Tibet. Collaboration and cooperation between U.S. and Nepalese seismologists is essential to this project, thereby contributing to improvements of Nepalese scientific infrastructure.
The newly collected, high quality NAMASTE array data will allow for a better understanding of the tectonic setting and rupture process of these large earthquakes and will both illuminate the processes causing large Himalayan earthquakes as well as large subduction zone earthquakes. Specifically, we will analyze the NAMASTE array data to address: 1) The geometry of the MHT fault zone, 2) the crustal composition in the MHT region, 3) the fault dynamics from aftershock characterization, and 4) the broader geotectonic setting. To address the geometry of the MHT, we will perform precise aftershock locations, develop a robust seismicity catalog from the NAMASTE and other regional arrays, and perform source characterization of aftershocks that will provide meaningful insights into the fault geometry, slip dynamics, source properties, and evolving state of stress during the aftershock sequence. To determine crustal composition, we will use local earthquake tomography and ambient noise tomography to derive crustal velocities and use known relationships between velocity and crustal composition and assess the presence and role of fluids in the crust, especially in the regions of active faulting. For fault dynamics, we will examine the spatiotemporal distribution of aftershocks and relation with rupture patches obtained by backprojection to allow for a better understanding of the rupture process and stress changes that occurred during the main shock and aftershocks. Finally, we will explore the broader geotectonic setting of the region by combining NAMASTE array data with other regional stations to derive tomographic velocity models, crustal and upper mantle anisotropy, and Moho and possibly lithospheric depths from receiver functions. We will further compare our seismology datasets with other geophysical and geological datasets (e.g., magnetotellurics, gravity, surface mapping, deformation) to derive the best possible understanding to date of the tectonics of the Himalaya and southern Tibet in Nepal and the surrounding countries. |